Louise Hoole will not relax until she has walked around the world, a seven-year mission to trace the steps of the ancient humans who made their way from Africa to South America. On Tuesday, months after an unplanned stay in intensive care in Botswana, she will rejoin her husband James Tremayne to pick up where they left off.

Ten months ago, our expedition looked like it might be defeated by so small a thing as a damaged tendon. Today things look very different.

The Footsteps of Man, as we call our mission, started in the Cape of Good Hope in May 2001. After 2,000km, we had to have a break while James had surgery on his ankle, but in November last year we started again, resuming our journey in the vast arid land of the Karoo in South Africa.

We loved it. But as we crossed the Orange River and moved up into the North West Province of South Africa we found ourselves in a very different place. Here was a rabidly racist province; one where apartheid was very much alive; a place with more farm murders than Zimbabwe: in short, a violent, nightmare land.

It was too dangerous to camp alone without putting ourselves under someone's protection and night after night - sometimes in town bars, more often on rural farms - our ears would be lashed by white men bragging of black men they had killed.

Louise carrying some of their gear in purpose-built carts

Some of the people I met there will haunt me forever. A fat white man telling us with lip-smacking relish of the black woman he had whipped "until her skin was stripy... like a zebra". The image of that same man later that night, when drink had done its work, weeping fat tears over the death of Princess Diana. "She was so beautiful..." he slurred, "so beautiful."

In Taung, 300 km from the Botswana border, I told James I couldn't take much more of it. For while it was true that I had to stop my lips curling at murderers' confessions, it had become second nature to say nothing. These were men not only with dangerous views but with a dangerous nature: Berettas nestled into fat groins and swastikas glinted between the white chest hairs of more than one of our hosts.

When we saw the blue, white and black Botswana flag it felt like life was looking up. For while it is true that Botswana is struggling with an HIV infection rate of 40%, the impression in Gabarone was of a joyous, well-dressed, well-nourished people.

Louise and James's planned route

I felt tired when I arrived but I put it down to the recent stresses of our journey. A few days later however James asked me why I was breathing "that way".

"What way?" I asked.

"Like an asthmatic old woman."

"I always breathe like this," I told him, "I'm perfectly all right." But a few weeks later, in the middle of the orange sand and dry air and mopane trees of Botswana, I collapsed.

Nothing can capture my terror at finding myself in intensive care - brain scans, ECGs, AIDS tests, malaria, sleeping sickness, lymphomas, MS. My wild fear (since we'd been bitten by spiders) was that one of those creatures had laid eggs in my brain; and from it, in a few weeks time, baby arachnids would hatch. The tests went on in Botswana for days. Then a doctor told me I was being airlifted to Johannesburg and thence to England: there was nothing more they could do for me here.

Strapped in a stretcher and covered with a red blanket I looked out of the aeroplane windows and savoured my last views off a Botswana I had hardly seen. I had told James before I left: "You'd better go on. If they tell me I'm going to die, I'll call and ask you to come back." Fortunately that phone call was never made.

James Tremayne has been walking on while Louise recovered

Week by week in England I have grown stronger, nourished both by my mother's soups and the satisfying sight of the pointer of James's progress moving slowly northwards through the vast continent of Africa. Last week he phoned me from Tanzania. His ankle has strengthened by adding a little cycling to his walking, and he's now clocked up 6,500 km. I'm now flying out to pick up our life together in Dar es Salaam.

I never knew it was possible to exhaust your adrenal gland or how listless it is possible to feel. "How long will you be away this time?" friends ask, "how long will it take to walk the rest of Africa... the world?" I tell them I don't know, that we gave up the itinerary long ago, that there are over 50,000 kilometres ahead.

I am ecstatic at the thought of seeing James after a separation of four months; joyful at the thought of a conversation that isn't strained by international lines and vast expense. And when I look back on what's happened - illogical though it may seem - I really believe those long months among men with swastikas had something to do with it all.

You can follow the progress of the mission, and see details of how you could support it, at the Footsteps of Man site, under Internet links.